1. Field of the Invention:
The present invention relates to excitation systems for synchronous dynamoelectric machines, and more particularly to means for supplementing the excitation provided by a pilot exciter in combination with a conventional brushless exciter.
2. Description of the Prior Art:
Brushless excitation systems are now widely used for supplying direct current field excitation to synchronous dynamoelectric machines such as large alternating current generators. Such brushless excitation systems include an alternating current exciter having a stationary field structure and a rotating armature member. A rotating rectifier assembly is carried on a common shaft with the exciter armature and is connected thereto to provide a direct current output. The output of the rectifier is connected to the field winding of the main generator which also rotates with the exciter armature and rectifier. In this way, an excitation system is provided which requires no sliding contacts.
In conventional arrangements, the main exciter for a synchronous dynamoelectric machine comprises an alternating current generator having its armature mounted on the same shaft as the field winding of the synchronous machine and also having a stator field winding which must be energized by direct current to create a magnetic field so that a voltage will be induced in a rotating armature of the exciter. In well known arrangements, the direct current excitation for the main exciter is provided by a pilot exciter having a permanent magnet rotor turned by a prime mover within an annular armature winding to produce excitation power for the main exciter.
The generated voltage of the main exciter depends upon the rate of change of flux linkages with the AC exciter armature winding and the AC exciter field winding. The fluxes depend on the reluctances of the iron portions of the magnetic circuits as well as of the air gaps of the main exciter. It will therefore be appreciated by those skilled in the art that magnetic saturation of the armature and stator core may appreciably influence the generated voltage of the exciter. The generated voltage of such an exciter may be characterized generally as a non-linear function of the field excitation. Because of this non-linear relationship, the armature voltage is a generally decreasing non-linear function of the armature current flowing in the armature winding. For most known applications, it is desirable that corrective measures be taken to compensate for the non-linear effects of the iron core saturation to provide a substantially constant armature voltage output which is independent of changes in the armature current of the exciter.
Regulating schemes are known for controlling directly the level of direct current excitation applied to the main exciter stator field winding. Such arrangements are generally responsive to changes of the time rate of change of the voltage and current in the output of the main turbine generator. Although such arrangements are generally satisfactory, there remains interest in providing improved regulating systems.